The present disclosure relates to the production of treated inorganic particles. While this disclosure can be used to produce a range of particles, advantageously, this disclosure provides a route to a durable grade pigment, without the necessity of depositing surface treatments on pigment particles by wet treatment methods.
U.S. Pat. No. 5,340,393 discloses processes for depositing dense amorphous silica coatings on inorganic particles, especially metal oxides via wet treatment methods. The inorganic particles include oxides of titanium, magnesium, calcium, barium, strontium, zinc, tin, nickel, and iron as well as complex oxides such as mica, cordierite, enorthite and pyrophyllite. The inorganic particles also include carbonates and sulfates such as those of calcium, barium and strontium.
Titanium dioxide is typical of an inorganic particle that is frequently treated prior to incorporation into end use applications. Titanium dioxide pigment particles, produced by either the chloride or the sulfate process, are processed in one or more wet treatment operations to deposit metal oxides on the surface of the pigment in order to optimize the pigment properties of dispersion, optical spacing or durability. Deposits of aluminum oxide or combinations of aluminum oxide and silicon dioxide, used alone or in combination with other oxides, are typical constituents of commercial titanium dioxide pigment. Such surface treatments are deposited through precipitation of the desired metal oxide in a wet chemical reaction. Thus, the base pigment, that is, the titanium dioxide particles produced at the exit point of the oxidizer in the chloride process or after calcination in the sulfate process, must be washed and processed through one or more wet treatment steps. Wet treatment is then followed by washing, drying and grinding to produce a product suitable for use in for example, exterior coatings and plastics products.
In the chloride process, an oxygen-containing gas is reacted with titanium tetrachloride (TiCl4) at temperatures ranging from 900° C. to 1600° C. in a vapor phase. The resulting gaseous suspension of TiO2 particles and free chlorine are discharged from the reactor and must be quickly cooled in a conduit, i.e., a flue, so that undesired TiO2 particle growth is prevented and particle agglomeration is minimized.
Processes to influence the titanium dioxide crystal formation and/or growth by addition of chemical agents in the oxidizer of the chloride process are taught in British Patent 689,123, and U.S. Pat. Nos. 3,856,929; 4,124,913; and 5,562,764.
So-called vapor or dry process to deposit surface treatments on the pigment in the oxidation step are taught in U.S. Pat. No. 4,050,951; PCT published patent application WO 96/36411; and European Patent 0 032 426. In U.S. Pat. No. 4,050,951 post treatment hydrolysis is taught. The disadvantage in this system is that the treatment step is a separate stage in the overall process, following oxidization that requires the separation of base pigment from the oxidation product, then grinding followed by hydrolysis at temperatures lower than those temperatures present in the oxidizer.
PCT application WO 96/36441 teaches a vapor phase treatment process requiring that the silicon tetrachloride addition must be made at a temperature of more than 1300° C. This application further teaches that the addition of metal halides can be made in any sequence and at any point in the reactor.
European Patent 0 032 426 teaches a post treatment of titanium dioxide particles in a fluid bed reactor. This process requires an activation step where the titanium dioxide particles are contacted with metal chlorides followed by a hydrolysis to convert residual chlorides to oxides and oxide hydrates.
U. S. Pat. Nos. 5,922,120 and 6,852,306 advance the technology of vapor or dry process to deposit surface treatments on TiO2 particles. These processes can produce durable TiO2 pigments with a very high percentage of particles treated. However, these processes rely on use of volatile silicon halides as the silica source for treating particles.
There is a need for a process to make a treated inorganic particle without use of wet treatment or volatile silicon halides, which may be difficult and/or hazardous to handle. In one particular example, there is a need for a process to produce a durable grade commercial titanium dioxide pigment of acceptable gloss and carbon black undertone (CBU) without use of wet treatment or volatile silicon halides. There is also a need to avoid the cost and additional processing required in the typical wet treatment operation. The present disclosure meets these needs.